Gandalf spoke of him as he told Frodo the true origin of his Ring. "It has been said that dragon-fire could melt and consume the Rings of Power, but there is not now any dragon left on earth in which the old fire is hot enough; nor was there ever any dragon, not even Ancalagon the Black, who could have harmed the One Ring, the Ruling Ring, for that was made by Sauron himself."[3]

Ancalagon's size is not specified, but may be gigantic due to the destruction of "the towers of Thangorodrim", which are elsewhere identified with the three smoking peaks of the mountain. He was however, said to be as big as a mountain.[6] However, his size is never directly specified and some powerful but small creatures are able to cause great destruction even in death, such as Durin's Bane breaking the slopes of Celebdil. Like all other Urulóki, Ancalagon breathed fire, which was said to be hotter than any other dragon's flame.

In 1977, an extinct genus of worms found in the The Walcott Quarry on Fossil Ridge in 1911 dating from the Middle Cambrian Era from the Cambrian Burgess Shale was named Ancalagon, inspired by Tolkien's dragon.[7]